"There's no logical explanation because two tug boats were moving the ship and there was a port pilot on board and sea conditions were optimal," the head of the Genoa Port Authority, Luigi Merlo, said.

The only thing left where the tower had stood was a leaning metal-framed stairway. Divers from the fire department joined the search for bodies.

The Jolly Nero, which is 781 feet long with a gross tonnage of 40,594 metric tons, is owned by local operator Ignazio Messina and Co.

"A thing like this has never happened, we are devastated," said Stefano Messina, one of the directors of the family-owned firm, who was in tears when he spoke to a local TV channel.